Is it normal to blur your vision voluntarily?

Since I was a kid, every once in awhile I would gaze at a stoplight or any source of light and make things blurry voluntarily; I can just make everything blurry by tightening my eye muscles or some unexplainable type of way that I can't really explain (focus/strain on the eyes maybe?). Anyway, I was staring at the moon last night, meditating if you will, and made my vision blurry for the heck of it and split the moon into 2 moons on my focal vision blurry mode (lol). Then I started to wonder if others can do the same, or if it's normal. I've been had this question for awhile but never really got around to asking anyone. Thanks for the help/advice/opinions.